Currently, as a next generation communication network NGN (Next Generation Network), intensive studies have been made on a core technology known as an IMS (IP Multimedia Subsystem) for flexibly providing multimedia applications including not only stationary communications and mobile communications but also audios, videos and the like by a packet communication network based upon the IP (Internet Protocol) communication protocol. See, for example, “IMS, a core of NGN” internet URL (http://web.forum.impressrd.jp/feature/) and Japanese patent publication (JP2006-115453A) for more information.
The IMS architecture comprises basic constituting elements as shown in FIG. 7. That is, FIG. 7 is a conceptual illustration of basic constituent elements of the IMS network. It comprises various multimedia terminals, i.e., user equipment UE (User Equipment) 201, 202 including mobile phones, stationary telephones, PDAs, notebook PCs, desktop PCs and the like as IMS clients, an IP network 500 including a plurality of IP networks such as a home network 100, another network (within the zone) 101 and the like, and application server group 300 such as application servers AS301, AS302, etc.
The IP network 500 interconnects the IMS clients, i.e., the user equipment UE 201, 202 using broadband networks such as ADSL, FTTH, etc. or wireless interfaces. Also, the IP network 500 interconnects the user equipment UE 201, 202 using the IP protocol for all communications, thereby enabling to commonly use the application servers AS301, AS302 for realizing various services. It is to be noted that the IP network 500 uses an SIP (Session Initiation Protocol) to control making or breaking of communication paths to be used for services. For this end, a plurality of SIP servers 100A, 100B, 101A and the like are installed in the IP network 500 so that they cooperate with one another.
In other words, the IMS communication provides a common system for providing various services (multimedia services) on the IP protocol based packet communication network. Using the IP protocol and interconnecting the user equipment UE 201, 202 that are provided with interfaces for IMS, the IP network 500 enables to perform communications using various media and make a connection to the application server AS301 for providing application services using audios and videos that the user chooses from the application server group 300 that provides various multimedia services.
Moreover, the IP network 500 allows the user equipment UE 201 to gain access to the home network 100 from another network 101 in his/her office for receiving the same services as from the home network 100 even if the user equipment UE 201 may move between networks. When moved to any other network, roaming is performed with the home network 100, thereby enabling to receive the same services as he/she is located in the home network 100. For this end, by the use of the system of performing the “home network control” that cooperates with the SIP server 101A in another network 101 at the location where he/she moves and the SIP servers 100A, 100B in the home network 100, the user is able to use always the same services whichever network he/she may move without depending upon the network at the location where he/she is.
Now, functions of the SIP servers 100A, 100B, 101A and the like as shown in FIG. 7 will be described in greater detail with reference to FIG. 8. FIG. 8 is a conceptual diagram to show functions of the SIP servers that are key constituent elements of the IP network 500 in the IMS architecture. The SIP servers comprise at least one of three kinds of functions. One is a P-CSCF (Proxy Call Server Control Function: proxy control function) for enabling the user equipment UE to gain access either directly or by way of an access network. One of the other functions is an I-CSCF (Interrogating CSCF: interrogating control function) for accepting any interrogation from other CSCF or having a gateware function with other networks. The last one is an S-CSCF (Servicing CSCF: call session control function) for performing a session control based upon the SIP in the home network. In other words, the P-CSCF function constitutes a user equipment accommodation session control server, the I-CSCF function constitutes an interrogating session control server and the S-CSCF function constitutes a call session control server for performing a core call session control in the IMS network.
Each of the P-CSCF, I-CSCF and S-CSCF functions may be configured as a physically separated server (i.e., a P-CSCF server, an I-CSCF server or an S-CSCF server) or alternatively one or more functions may be physically merged with another as an integrated server. Moreover, it is also possible to configure a single server having a plurality of divided sections for the same function so as to split the loads.
Each of the SIP servers 100A, 100B and 101A as exemplified in FIG. 7 is provided with either one or more of these functions. In the particular example as shown in FIG. 8, the SIP server 100A in the home network 100 is provided with a P-CSCF function 10A for connecting the user equipment UE 202 and a single S-CSCF function 20A. The SIP server 100B is provided with a pair of S-CSCF functions 20B, 21B and an I-CSCF function 30B for making connection with another network 101 in which the user equipment UE 201 is moved. On the other hand, the SIP server 101A in another network 101 is provided with a P-CSCF function 11A for connecting the user equipment UE 201 and an I-CSCF function 31A for making connection with the home network 100. As described hereinbefore, each SIP server is provided with a plurality of CSCF functions and it is also possible to provide independent servers depending upon traffic amounts just like in this particular example.
Moreover, as shown in FIG. 8, disposed in the IP network are an HSS server (Home Subscriber Server) 40 and, if necessary, an SLF function (Subscriber Locator Function) 50 as a database server. The HSS server 40 is a database server for registering and saving such information as user information, profiles on services that each user subscribes, user authentication information, user movement management information (current location information) and the like, thereby registering all information that are required when each CSCF function performs session control. On the other hand, the SLF function 50 is a database for registering the information to show the relationship between the HSS server 40 in which user information are registered and the users. If there are plural HSS servers within the network, the SLF function 50 has a function to receive interrogations from the I-CSCF function and conduct a search for a particular HSS server in which the user information for the interrogated user is registered.
Now, detailed descriptions will be given hereunder on each function of the P-CSCF function, the I-CSCF function and the S-CSCF function.
The P-CSCF function (Proxy Session Control function or User Equipment Accommodation Session Control Function) connects an IMS client or the user equipment UE by way of a broadband subscriber network such as ADSL, FTTH or the like or any other access network such as a wireless network, a PSTN network or the like for receiving SIP messages from the user equipment UE or transmitting or sending SIP messages to the user equipment UE. In case of connecting by way of the access network, a connection is made from the existing W-CDMA network for accommodating, for example, a mobile phone user equipment UE by way of a packet switching known as a GGSN (Gateway GPRS Support Node) that has a function of connecting to an external network. On the other hand, any call from the broadband access network such as fixed ADSL, FTTH or the like or from the wireless LAN is connected by way of a relay router in the IP network of the access system.
The P-CSCF function is allocated at the registration (location registration) time of the user that is performed prior to start of IMS communication. After completing authentication between the user equipment UE and the IMS communication IP network, an IPsec tunnel is set between the user equipment UE for maintaining communication secrecy. All SIP messages that are transmitted or received between the user equipment UE are transferred safely over the IPsec tunnel and rationality check of the received SIP messages is also made in the P-CSCF function. Not only the P-CSCF function within the home network but also roaming is performed using the P-CSCF function in another network at the location where the user is moved in, thereby enabling to effectively transfer the traffic from the user equipment UE to the network at the moved location. However, it is to be noted that tunneling is made on all traffics such as VoIP or the like from the user equipment by the P-CSCF function in the home network if no IMS function is included in the network at the moved location.
The I-CSCF function (Interrogating Session Control Function) is disposed in the home network. At the registration time of the user equipment UE to the IMS communication IP network that is performed prior to start of communication, it identifies the HSS server in which the user information (i.e., subscriber information) of the user equipment UE is registered by making a reference with the SIP messages received by the P-CSCF function or by gaining access, if necessary, to the SLF function (i.e., Subscriber Locator Function) if there are plural HSS servers (i.e., Home Subscriber Servers) in which the user information (subscriber information) is registered. Then, a decision is made of the S-CSCF function that performs the call session control of the user equipment UE in response to the information from the HSS server. The SIP messages of the user equipment UE that are received from the determined P-CSCF function are routed for succeeding the registration procedures for the user equipment UE.
Furthermore, the I-CSCF function determines the S-CSCF function that performs the call session control for the user equipment UE by gaining access to the HSS server in which the user information (subscriber information) for the user equipment UE is registered even if accessed through any other network, thereby succeeding communication from the other network. This means that the I-CSCF function enables to smoothly split loads in case of existence of plural HSS servers and simultaneously plays a role of shielding the structures within the network from outside networks.
The S-CSCF function (Call Session Control Function) is a SIP server that is a core of the session control by the SIP protocol as a SIP registration server that performs the call session control of the user equipment UE utilizing the registered IMS services and holds the user information (subscriber information) and the current location information of the user equipment UE that are downloaded from the HSS server. It also performs the session control of the communications between user equipment UE and activates the service procedures of the application server AS in response to the request of the user equipment UE in accordance with the user information (subscriber information) setting that is registered in the HSS server. Moreover, when the user equipment UE appoints with whom to communicate by its telephone number based upon the E.164 Recommendation, it has a function to make the routing based upon such telephone number.
The S-CSCF function makes a connection to a common enabler (general purpose functions to be used to realize application services such as presences, messages or the like) to be commonly used for various different application services and the application servers AS for performing controls for individual application services by way of a standard interface known as the SIP based ISC (IMS Service Control).
On the other hand, the application server group 300 including a plurality of AS (Application Servers) 301, 302, . . . as shown in FIG. 7 are a group of servers for processing various application services. They are activated by the S-CSCF function in response to the processing request from each user equipment UE. It is to be noted that applications for services similar to the conventional telephone network are also provided.
Now, basic transmission (i.e., sending) and receiving procedures in case when the IMS user performs IMS communications will be described hereunder with reference to FIGS. 9 and 10. FIG. 9 is a flowchart to show the flow of transmission (or calling) procedures when the user equipment UE performs IMS communications. On the other hand, FIG. 10 is a flowchart to show receiving procedures when the user equipment UE performs IMS communications. In the following descriptions, the servers for performing the P-CSCF function, the I-CSCF function and the P-CSCF function are referred to as the P-CSCF server, the I-CSCF server and S-CSCF server, respectively.
Firstly, the transmission or calling procedures will be described with reference to FIG. 9. The user equipment UE uses an “INVITE message” in order to start a session. The user equipment UE sets the URI (Uniform Resource Identifier) for the S-CSCF server that are stored at the registration time prior to starting IMS communications as the “Service -Route” header in the “INVITE message” for instructing the routing to the user equipment UE (sequence SQ21). The P-CSCF server that received the “INVITE message” from the user equipment UE makes a reference with the S-CSCF server that is memorized at the registration time. If agrees, the received “INVITE message” is transferred to the S-CSCF server without gaining access to the I-CSCF server (sequence SQ22).
The P-CSCF server that received the “INVITE message” from the I-CSCF server determines the routing for transmitting the “INVITE message” based upon the information with whom the communication is made as designated in the “INVITE message” (sequence SQ23). Subsequently, the S-CSCF server receives a tentative response “100 Trying” that indicates to have received the “INVITE message” from the subsequent CSCF at the receiving side (sequence SQ24). When receiving a tentative response “180 Ringing” that indicates ringing (sequence SQ26), it is transferred to the respective transmission (calling) side user equipment UE (sequence SQ25 and sequence SQ27).
The transmission side user equipment UE that received the tentative “180 Ringing” or the S-CSCF server that received a final response “200 OK” that indicates the session setting completion is received from the subsequent CSCF side (i.e., the receiving side user equipment UE in the particular example in FIG. 9) (sequence SQ28) transfers the received “200 OK” to the transmission side (i.e. precedent transmission side in FIG. 9) (sequence SQ29). When an acknowledgement response “ACK” to the “200 OK” is received from the transmission side (sequence SQ30), the S-CSCF completes the set-up procedures for opening the session and sends the acknowledgement response “ACK” to the transmitter of the “200 OK” (sequence SQ31), thereby establishing the transmission side communication session of the IMS communication in accordance with the SIP protocol.
The “INVITE message” in accordance with the SIP protocol adopts means for confirming reliable data transmission to the communication partner by three directional handshakes of “request (INVITE)”, “final response (200 OK)”, and “acknowledgement (ACK)”. Moreover, a tentative responses of “1 xy” (100 Trying and 180 Ringing) is returned from the receiving side to the transmission side before returning the “200 OK”, thereby notifying the progress of the session set-up.
It is to be noted that the “INVITE message” and the “200 OK” (success message of session set-up request) in the IMS communication use the protocol known as an “SDP (Session Description Protocol)” that describes the contents of the established multimedia session.
Now, the receiving procedures will be described hereunder with reference to FIG. 10. The “INVITE message” from the S-CSCF server at the transmission or calling side is received by the I-CSCF server (sequence SQ41). The I-CSCF server interrogates the HSS server about the S-CSCF server allocated thereto at the registration time of the user equipment UE at the receiving side using the Diameter protocol and routing of the received “INVITE message” is made to the URI for the S-CSCF server that is returned from the HSS server (sequence SQ42).
The S-CSCF server that received the “INVITE message” distinguishes the URI for the P-CSCF server that is recorded at the registration time based upon the information set in the Path header and routing of the “INVITE message” is made to the P-CSCF server (sequence SQ43). Then, the P-CSCF server transfers the “INVITE message” to the user equipment UE that is designated in the received “INVITE message” (sequence SQ44) and returns to the S-CSCF server at the receiving side by way of the S-CSCF server and the I-CSCF server the tentative response “100 Trying” to indicate that the “INVITE message” has been distributed to the user equipment UE (sequence SQ45).
The user equipment UE that received the “INVITE message” goes to the ringing state for calling the user and returns the tentative response “180 Ringing” to the P-CSCF server (sequence SQ46). The P-CSCF server that received the tentative response “180 Ringing” indicating to be called by the user equipment UE transfers the tentative response “180 Ringing” to the S-CSCF server at the receiving side by way of the S-CSCF server and the I-CSCF server (sequence SQ47).
Thereafter, when the user called from the user equipment UE responds, the user equipment UE transmits the final response “200 OK” to the P-CSCF server indicating that the request designated in the “INVITE message” has been accepted (sequence SQ48). The P-CSCF server that received the final response “200 OK” from the user equipment UE indicating that the request was accepted transfers the final response “200 OK” to the S-CSCF server at the transmission side by way of the S-CSCF server and the I-CSCF server (sequence SQ49). Since the final response “200 OK” is returned in a form of including the URI for the CSCF servers other than the I-CSCF server sequentially inserted into the “Record-Route” header at the time of receiving the “INVITE message”, all subsequent messages will be directly transmitted without any intervention of the I-CSCF server.
Thereafter, acknowledgement responses “ACK” of the transmission side to the returned final response “200 OK” are directly transmitted to the S-CSCF server at the receiving side from the S-CSCF server at the transmission side without any intervention of the I-CSCF server (sequence SQ50). The S-CSCF server at the receiving side that received the acknowledgement response “ACK” from the S-CSCF server at the transmission side transmits the acknowledgement response “ACK” to the user equipment UE at the receiving side byway of the P-CSCF server (sequence SQ51). As a result, established is the receiving side communication session by the IMS communication in accordance with the SIP protocol.
Now, basic procedures for terminating the IMS communication will be described with reference to FIG. 11. FIG. 11 is a sequence chart to show the flow of processing that the user equipment terminates the IMS communication. The User equipment UE for terminating communication and releasing the session transmits a “BYE message” that means the end of session to the communicating user equipment UE through the route of the P-CSCF server and the S-CSCF server at the transmission side as well as the S-CSCF server and the P-CSCF server at the receiving side that are used in the IMS communication (sequence SQ61).
The receiving side user equipment UE that received the “BYE message” returns a final response “200 OK” indicating that the release of session is accepted to the “BYE message” transmitting user equipment UE in the opposite direction to the transmission route of the “BYE message” (sequence SQ62). As a result, the communication session is released. Different from the “INVITE message”, the “BYE message” adopts a two-directional shake-hand system of the “request (BYE)” and the “200 OK”.
If the user equipment UE uses an application service that the application server AP provides, such set-up is made at the registration time of the IMS communication and a “Filter Criteria” that is a reference of judgment of the application service is returned to the S-CSC server from the HSS server. Examples of the “Filter Criteria” include, for example, types of message, direction of session, kinds of registration (initial registration, re-registration and erase of registration), presence or absence and contents of the SIP header, SDP (Session Description Protocol) parameters (kinds of medium) and the like. Also included is designation information of the application server AS that is included in the routing of the SIP message.
It is to be noted that the use of the application server AS enables to provide telephone services similar to the line switching such as a call transferring service or the like other than music and video distribution services such as, for example, “karaoke” services and VoD (Video on Demand) services.
When SIP messages from the user equipment UE are received/ the S-CSCF server performs session control to transfer to the application server AS only SIP messages that satisfy the “Filter Criteria” by referring to the “Filter Criteria” that was received from the HSS server at the registration time of the IMS communication. In this way, SIP messages that are transferred to the application server AS are able to be transferred from the application server AS to the distribution addresses for providing the corresponding services.
As for identifiers for identifying the kind of services, used is a “public service ID (PSI)” similar to the “public user ID (IMPU)” for identifying each subscriber. By designating a desired PSI by the user equipment UE, the S-CSCF server directly routes the messages to the application server AS corresponding to the designated PSI, thereby enabling to provide the application services.
In the IMS architecture having the network construction as described hereinabove, it is required that the user equipment UE performs proceedings (i.e., registration request by REGISTER message) to determine the S-CSCF server for making a call session control for each user equipment UE among a plurality of S-CSCF servers (call session control servers) disposed in the home network of each user equipment UE prior to transmission and receiving operations of each user equipment UE using the SIP protocol.
At this time, in a prior art, the P-CSCF server that received the REGISTER message from the user equipment UE for requesting registration interrogates the I-CSCF server. Then, the I-CSCF server that received the interrogation causes the HSS server to transfer a list of all S-CSCF servers in the home network and determines the S-CSCF servers in charge of the user equipment UE that sends the registration request, i.e., the S-CSCF servers that administrate call session controls of the user equipment UE in a predetermined sequence or order.
However, since the registration status of the user equipment UE dynamically changes in each S-CSCF server due to not only the registration request of the user equipment UE but also erasing of such registration, the conventional method of allocating the S-CSCF servers in a predetermined sequence results in incapability of evenly distributing the number of user equipment UE that are registered in the S-CSCF servers, thereby unavoidably encountering a trouble of concentrated loads in a particular S-CSCF server.